In resource recovery, it may be useful to monitor the fluid level within a pipe at any level of a borehole that has been drilled either for exploratory or production purposes, for example, in dual gradient drilling applications. For instance, the fluid level within the pipe can be an indicator of the pressure seen by the lower end of the borehole. Generally, pressure will lift, or stabilize, the fluid within the pipe to a level such that the hydrostatic column of fluid balances the pressure being applied to the lower end of the pipe. Knowing the actual height of the fluid and the density or weight of the fluid allows a user to readily determine the hydrostatic pressure of the system. Typically, if the hydrostatic pressure is greater than the pressure exerted by the surrounding formations then positive hydrostatic pressure occurs. When a well is drilled using rotary drilling methods, it is customary to maintain the weight of the drilling fluid at a weight that gives positive hydrostatic pressure. The advantage of positive hydrostatic pressure is that the formation fluids do not escape.
During production, the fluid level of the well is typically measured using a device such as an echometer. Generally, the echometer sends a pressure pulse generated by a compressed air source or by an explosive charge into a pipe. The sound wave that is generated travels through the pipe and is reflected at the fluid interface. However, the use of the echometer generally requires that the pipe be filled with a gas that can transmit sound waves. In some drilling applications, having a pipe downhole filled with gas is not a feasible option since the pipe may be sealed from the environment and may only contain a vacuum, thus rendering the echometer unusable.